We will study the proteins of the platelet plasma membrane to characterize the molecular basis of platelet contact interactions. Although platelet adhesion is the primary event in both hemostasis and thrombosis, little is known about the effects of cell aging and contact interactions on plasma membrane surface glycoproteins. We have developed a high specific activity, nonpenetrating radioactive label, 125I-diazotized diiodosulfanilic acid (DD125ISA), and have demonstrated that it specifically binds to all major exposed glycoproteins of the plasma membrane of human and rabbit platelets. During circulation in rabbits, membrane-bound DD125ISA is lost from platelets more rapidly than the cytoplasmic label, 51Cr, and the DD125ISA loss is proportional among the three membrane glycoproteins, suggesting loss of fragments of membrane. We have postulated that this could occur during contact interactions in the circulation. Using methods to quantify platelet membrane proteins by SDS-polyacrylamide gel electrophoresis and immunoelectrophoresis in addition to our DD125ISA label, our investigations will (1) further define the protein structure of the platelet plasma membrane, (2) assess changes in platelet membrane proteins during cell aging in vivo and storage in vitro, (3) characterize the molecular alterations of membrane proteins from patients with hemostatic disorders, and (4) identify the surface proteins involved in platelet contact interactions with collagen and damaged endothelium. These studies should improve the understanding of the relationship between events of hemostasis and thrombosis and the membrane proteins of circulating platelets.